travis: Remove deprecated Xcode images from build
Created by: femtobit
The Travis CI images for Xcode 8.1 and 8.2 have been retired [1]. According to a message in the Travis web interface, they are now routed to the Xcode 8.3 image [2]. Therefore this commit removes them from the build configuration.
[1] https://blog.travis-ci.com/2017-11-21-xcode8-3-default-image-announce [2] "This job was configured to run on an OS X image that was retired on November 28, 2017. It was routed to our Xcode 8.3 image."